1. Field of the Invention
The present invention relates to a burst mode optical receiver, and more particularly to peak and bottom detectors in a burst mode optical receiver.
2. Description of the Related Art
In order to more rapidly transmit more information to subscribers, next generation communication systems have required a fiber to the home (hereinafter, referred to as FTTH), in which an optical line is provided in each home. However, the FTTH has financial deployment limitations, in that the cost to replace existing subscriber copper wire networks are significant. To lower costs, passive optical networks (hereinafter, referred to as PONs) have been considered in constructing an FTTH.
The PON includes an optical line termination (hereinafter, referred to as OLT) in a central office, an optical splitter and one or more optical network unit (hereinafter, referred to as ONU) corresponding to subscribers.
When communication from one OLT to a plurality of ONUs is performed, data is transmitted downward simultaneously. When a plurality of ONUs transmit data to the OLT, a time division multiple access (TDMA) technique is used in order to avoid a collision between signals transmitted by each ONU. Further, since each ONU has a different distance from the OLT, each signal transmitted from the ONUs with the same optical output has different optical power in the angle of the receiver of the OLT. Consequently, the receiver of the OLT requires a burst mode optical receiver to process signals with the various optical powers.
FIG. 1 is a circuit diagram of a burst mode optical receiver using a double feedback structure. Referring to FIG. 1, the burst mode optical receiver 60 with the double feedback structure includes a photodiode 62 for converting an optical signal into an electrical current signal proportional to the optical signal, a gain controllable amplifier (hereinafter, referred to as GCA) 64 for amplifying the current signal outputted from the photodiode 62 to a voltage signal, an automatic gain control loop 66 for adjusting an amplifier's gain from a differential output of the GCA 64 by means of a feedback loop and a DC offset control loop 68 for adjusting DC offsets of two differential output signals from the differential output of the GCA 64. The burst mode optical receiver 60 with the feedback structure described above may obtain satisfactory performance/characteristics. However, the feedback structure generally requires quite a long time for feedback stability. Therefore, the feedback stability time may be limitation factor to rapid operation of the burst mode optical receiver. Further, the design of the feedback structure by nature requires a high degree of complexity.
In order to improve such a burst mode optical receiver having the feedback structure, a feed-forward structure was proposed. The burst mode optical receiver with a feed-forward structure is further described in “an instantaneous response CMOS optical receiver IC with wide dynamic range and extremely high sensitivity using feed-forward auto-bias adjustment, IEEE J. Solid state circuit, VOL. 30, NO. 9, pp. 991˜997, September 1995”.
FIG. 2 is a circuit diagram of a burst mode optical receiver having a feed-forward structure. Referring to FIG. 2, the burst mode optical receiver 70 with the feed-forward structure includes a photodiode 72 for converting an optical signal into an electrical current signal proportional to the optical signal, a gain controllable amplifier (hereinafter, referred to as GCA) 74 for amplifying the current signal outputted from the photodiode 72 to a voltage signal. An output of the GCA 74 is connected to a limiting amplifier 79 located behind of the burst mode optical receiver 70 in order to convert an analog signal into a signal with a digital level.
In order for the limiting amplifier 79 with a differential input structure to reproduce a signal with a 50% duty cycle, the output of the GCA 74 and a reference voltage of the outputted signal are necessary. An automatic threshold controller (hereinafter, referred to as ATC) 80 finds a reference voltage. This reference voltage is an intermediate voltage value of an outputted signal, of the signal outputted from the GCA 74 and enables the reference voltage to be a reference voltage source of the limiting amplifier 79. The ATC 80 finds a peak value and a bottom value of the signal from the signal outputted from the GCA 74. Then, it outputs an intermediate value which is an average value of the peak value and the bottom value. Accordingly, a peak detector 76 and a bottom detector 78 are necessary for the ATC 80. Further, the ATC 80 includes a pair of resistors R1 and R2 for distributing a voltage of a signal passed through the peak detector 76 and the bottom detector 78. The resistors R1 and R2 have a resistance value for generating an intermediate voltage {Vref=(Vpeak+Vbottom)/2}, that is, a reference voltage (Vref), of the peak voltage and the bottom voltage.
FIG. 3 is a circuit diagram of a peak detector used in a conventional burst mode optical receiver. FIG. 4 is a circuit diagram of a bottom detector. Referring to FIG. 3, the peak detector 90 includes an amplifying terminal 92 for reducing an offset of a peak value, a NPN transistor 94 for functioning as a diode and a peak hold capacitor 96 for charging a peak value. When a burst signal applied through the amplifying terminal 92 rises up to a peak value, the NPN transistor 94 is turned on and the peak hold capacitor 96 is charged up to a peak value.
Further, referring to FIG. 4, a bottom detector 100 has a similar construction to the peak detector. It includes an amplifying terminal 102 for reducing an offset of a peak value, a PNP transistor 104 for functioning as a diode and a peak hold capacitor 106 for charging a bottom value. The bottom detector 100 has a different type of transistor for functioning as a diode in comparison with the peak detector 90 and a direction of diode changes according to the transistor type.
In the operation of the bottom detector 100, when a burst signal applied through the amplifying terminal 102 falls up to a bottom value, the PNP transistor 104 is turned off and the peak hold capacitor 106 is charged up to a bottom value.
Charge time required for rising or falling up to the peak value and the bottom value becomes an important factor in detecting the burst signal, when the peak value and the bottom value of the burst signal are detected in the peak detector 90 and the bottom detector 100 in the burst mode optical receiver.
Conventional peak detectors and bottom detectors have a relatively fast charge time with respect to a small input signal and a relatively slow charge time with respect to a signal with a large input size. Further, the charge time may be fast with respect to a small input signal. However, an overcharge may occur, and thus causing fluctuations. Accordingly, since charge time of an input signal is closely connected with performance of the peak detector and the bottom detector, adjustment of the charge time becomes a very important factor.